I definitely think people voted illegally (a woman admitted to voting for Trump twice) but definitely not enough to swing the popular vote.
Sure, there have been a few illegal votes. This should certainly be caught when possible and the offender punished; but there have been only a handful of isolated occurrences ever reported. There certainly haven't been nearly enough to change the result of anything but a razor-thin election.
What's worrisome is the possibility, or even deliberate attempt, to make voting harder in the name of stopping illegal votes. Voting is the bedrock of our form of government, so it should be made as easy as possible for those who are eligible to vote. Some people say that voting should be difficult to show that a person is serious about it, but I have nothing but contempt for this viewpoint.
All citizens -- even those who don't put much thought into it -- have the right to a say in their representation.
I'm OK with requiring a photo ID for voting, but
only if the ID is free and easy to get (i.e., you don't have to travel 50 miles to the next county). If you have to pay for the necessary ID, it becomes in effect a poll tax, which is prohibited by the 24th Amendment.